Industrial waste.
If they ever have a contest for most beautiful industrial waste, these nickel crystals will win hands down. Yes, that's right, they are industrial waste, packed up and shipped out to the landfill by the barrel full, or so I am told. These were given to me by a former employee of the Flex-n-Gate company, the largest maker of automobile bumpers in the US. (The company also donated my very first nickel sample.)

These crystals grow slowly on the racks used to hold bumpers in the electroplating solution: Where a flaw develops in the insulation one of these starts to grow, and it just keeps growing until someone notices it and whacks it off with a hammer or something. Must be a real nuisance.

I made several 3D rotatable images of individual crystals. Here are three more in addition to the one above:, , ,

I've listed the source as anonymous, because I'm not entirely sure how the company feels about employees taking these things home. They are normally collected and sold for their scrap value. They are, according to x-ray fluorescence spectroscopy, about 96% nickel, the remainder mostly chromium with perhaps a small amount of iron and copper. But this figure is probably misleading: Calculations based on what I now know the process to be indicate that they should be at least 99.9% nickel: The XRF number is probably mislead by the surface being mostly chromium
Photographs do not do these things justice: You really have to see one to appreciate just how shiny it's possible for an object to be.

More recently (late 2005) I've acquired 200 pounds (!) of these sort of nodules above-board by purchasing them from the company at the same price they would have sold them to a recycler for. The batch is of course mixed, but the great bulk of it is crystals like this ranging in size from 1/2" to several inches across.